Compression of Encrypted Video

National Science Foundation CCR-0330514, National Science Foundation CNS-0519354 and Agency for Science, Technology and Research

Coding schemes for secure and efficient communication over noiseless public channels traditionally compress and then encrypt the source data. In networks with inconsistent and unpredictable link bandwidths, reversing the ordering of compression and encryption would be useful and even necessary for the efficient distribution of protected media content. The ability to compress encrypted data would allow for local adaptations of data sizes based on link availability. Indeed, not only is it possible to reverse the order (i.e., compress encrypted data), but under some conditions neither security nor compression efficiency need be sacrificed. To achieve compression in practice, it is crucial that our models are well-matched to the underlying source and are compatible with our framework. We have studied various models for the source data, including 1-dimensional and 2-dimensional Markov models. We used these models to develop practical codes for the compression of encrypted "real-world" data sources such as text, images, and video. For comparison, consider that a state-of-the-art motion-compensated lossless video encoder can compress each unencrypted frame of the standard "Foreman" test video sequence by about 59%. The algorithms we develop can compress the same frames, even after encryption, by about 33%.